The present application relates to a negative electrode suitable for use in a lithium secondary cell or the like, and to a secondary cell. More particularly, the application relates to improvements in first-time discharge capacity and charge-discharge cycle characteristics of a secondary cell.
In recent years, mobile apparatuses have been being enhanced in performance and in the number of functions. Attendant on these trends, secondary cells for use as power sources in mobile apparatuses are required to be smaller in size, weight and thickness, and are keenly desired to show an enhanced capacity.
As a secondary cell capable of meeting these requirements, there is known the lithium ion secondary cell. The cell characteristics of the lithium ion secondary cells vary greatly depending on the electrode active materials used, and the like. In the representative lithium ion secondary cell put to practical use at present, lithium cobalt oxide is used as the positive electrode active material, and graphite is used as the negative electrode active material. The cell capacity of the lithium ion secondary cell with such a configuration is approaching the theoretical capacity, and it is difficult to largely enhance the capacity by improvements in the future.
In view of this, it is being investigated to realize a considerable enhancement of the capacity of the lithium ion secondary cell by a method in which silicon, tin or the like element capable of alloying with lithium at the time of charging is used as a negative electrode active material. However, where silicon or tin or the like is used as a negative electrode active material, expansion and contraction of the negative electrode active material layer attendant on the charging and discharging will be large. Due to the expansion and contraction attendant on charging and discharging, therefore, the active material may be pulverized or come off the current collector, with the result of a lowering in cycle characteristics.
To solve this problem, negative electrodes have been proposed in recent years in which a negative electrode active material layer of silicon or the like is laminated on a negative electrode current collector (refer to, for example, Japanese Patent Laid-open No. Hei 8-50922, Japanese Patent No. 2948205, and Japanese Patent Laid-open No. Hei 11-135115). It is said that, with such a configuration, the negative electrode active material layer and the negative electrode current collector are firmly bound to each other in integrity, so that the active material can be restrained from being disintegrated due to the expansion and contraction attendant on charging and discharging. In addition, the electronic conductivity in the negative electrode is enhanced effectively.
Besides, Japanese Patent Laid-open No. 2002-83594 (pp. 11 to 13) describes that the negative electrode current collector on which the negative electrode active material layer of silicon or the like is to be laminated is formed from a metal capable of alloying with the negative electrode active material, from the viewpoint of enhanced adhesion to the negative electrode active material. It is also described that in the case of laminating a silicon and germanium layer, copper is particularly preferable for use as the material of the current collector. It is further described that the copper foil for this purpose is preferably an electrolytic copper foil having a high surface roughness Ra. The electrolytic copper foil is a copper foil obtained, for example, by a method in which a metallic drum is immersed in an electrolytic solution containing copper ions, a current is passed in the solution while rotating the drum, to deposit copper on the surface of the drum, and the thus deposited copper film is peeled, to obtain the desired copper foil. The surface(s) of the electrolytic copper foil can be roughened by depositing copper particulates on one side or both sides of the copper foil by an electrolytic process.
However, even in the case of the negative electrode in which the negative electrode active material layer and the negative electrode current collector are integrated and which is produced by a specially contrived method as above-mentioned, repetition of charging and discharging brings about severe expansion and contraction of the negative electrode active material layer, whereby stresses are exerted on the current collector. As a result, the electrode inclusive of the current collector may be deformed or collapsed, making it impossible to obtain sufficient cycle characteristics.